1. Field of the Invention
The present invention relates to a variable-shape mirror that permits the shape of a mirror surface thereof to be varied, and more particularly relates to the amount of shape variation of the mirror surface of a variable-shape mirror. The present invention also relates to an optical pickup device provided with a variable-shape mirror.
2. Description of Related Art
When information is read from or written to an optical disc such as a CD (compact disc) or DVD (digital versatile disc) by the use of an optical pickup device, the relationship between the optical axis of the optical pickup device and the disc surface should ideally be perpendicular. In reality, however, when the disc is rotating, their relationship does not always remain perpendicular. As a result, with an optical disc such as a CD or DVD, when its disc surface becomes inclined relative to the optical axis, the optical path of laser light is so bent as to produce coma aberration.
When coma aberration is produced, the spot of laser light shone on the optical disc deviates from the proper position, and, when the coma aberration becomes larger than permitted, inconveniently, it becomes impossible to accurately write or read information. For this reason, there have conventionally been proposed methods for correcting coma aberration as described above and other aberrations by the use of a variable-shape mirror.
For example, JP-A-2004-70004 proposes a method of reducing wavefront aberrations in a variable-shape mirror employing piezoelectric elements and having a unimorph or bimorph structure. The variable-shape mirror here has, for example, a structure as shown in FIGS. 6A, 6B, and 6C. FIG. 6A shows the variable-shape mirror with the mirror fitting member 108 thereof removed, as seen from the side opposite to the mirror material 101 thereof. FIG. 6B is a sectional view as cut along line A-A shown in FIG. 6A. FIG. 6C is a plan view as seen from the side of the mirror material 101. Reference numeral 102 represents piezoelectric elements, 103 represents wiring electrodes, 104 represents individual electrodes, 105 represents a mirror base, 106 represents a fixed-shape portion, 107 represents slits, and 109 represents a support portion.
In this structure, when, with the wiring electrodes 103 grounded, a positive voltage is applied to one of the individual electrodes 104 and a negative voltage to the other, one of the piezoelectric element 102 contracts and the other expands. Thus, the mirror surface becomes convex in the part thereof located on one side of the center thereof in the A-A direction and concave in the part thereof located on the other side. It is described that using this variable-shape mirror in an optical pickup device helps reduce wavefront aberrations. Moreover, JP-A-2004-70004 also proposes designing the support portion 109 of the variable-shape mirror structured as described above so that its shape can be elastically varied. This helps enhance the shape variation efficiency of the mirror portion. Here, the mirror portion denotes the part including the mirror material 101 and the mirror base 105. The support portion 109 denotes the part of the mirror portion located between the variable-shape part, which is the central part of the mirror portion including the part where the piezoelectric elements 102 are provided, and the fixed-shape portion 106, and thus denotes the part of the mirror portion that supports the variable-shape part with respect to the fixed-shape portion.
JP-A-2004-347753 also proposes an optical pickup device that can correct wavefront aberrations by the use of a variable-shape mirror. Here, it is proposed to provide an elastic plate diaphragm in the variable-shape mirror provided with a piezoelectric diaphragm and to give elasticity to part of the components. This helps realize a variable-shape mirror that offers a wide shape variation range with application of a low voltage.
In correcting aberrations with a variable-shape mirror, to achieve a given degree of aberration correction, the variable-shape mirror is required to be able to greatly vary the shape of the mirror portion thereof. In this respect, with the variable-shape mirrors disclosed in JP-A-2004-70004 and JP-A-2004-347753 mentioned above, although it is proposed to enhance the shape variation efficiency of the mirror portion for a given applied voltage, the amount of expansion and contraction of the piezoelectric elements themselves is so extremely small that enhancing the shape variation efficiency of the mirror portion for a given applied voltage does not much contribute to increasing the amount of shape variation of the mirror portion.
The amount of expansion and contraction of piezoelectric elements can be increased by laying piezoelectric elements in multiple layers, but there is almost no room for enlarging a variable-shape mirror incorporated in an optical pickup device. Thus, there have been demands for variable-shape mirrors that permit the amount of shape variation of the mirror portion thereof to be increased without increasing the external dimensions.